Network Working Group B. Campbell, Ed.
Request for Comments: 3428 J. Rosenberg
Category: Standards Track dynamicsoft
H. Schulzrinne
Columbia University
C. Huitema
D. Gurle
Microsoft Corporation
December 2002
Session Initiation Protocol (SIP) Extension for Instant Messaging
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
Instant Messaging (IM) refers to the transfer of messages between
users in near real-time. These messages are usually, but not
required to be, short. IMs are often used in a conversational mode,
that is, the transfer of messages back and forth is fast enough for
participants to maintain an interactive conversation.
This document proposes the MESSAGE method, an extension to the
Session Initiation Protocol (SIP) that allows the transfer of Instant
Messages. Since the MESSAGE request is an extension to SIP, it
inherits all the request routing and security features of that
protocol. MESSAGE requests carry the content in the form of MIME
body parts. MESSAGE requests do not themselves initiate a SIP
dialog; under normal usage each Instant Message stands alone, much
like pager messages. MESSAGE requests may be sent in the context of
a dialog initiated by some other SIP request.
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Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY" and "OPTIONAL" are to be interpreted as described
in BCP 14, RFC 2119 [6] and indicate requirement levels for compliant
SIP implementations.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Scope of Applicability . . . . . . . . . . . . . . . . . . . 3
3. Overview of Operation . . . . . . . . . . . . . . . . . . . 4
4. UAC Processing . . . . . . . . . . . . . . . . . . . . . . . 5
5. Use of Instant Message URIs . . . . . . . . . . . . . . . . 6
6. Proxy Processing . . . . . . . . . . . . . . . . . . . . . . 6
7. UAS Processing . . . . . . . . . . . . . . . . . . . . . . . 7
8. Congestion Control . . . . . . . . . . . . . . . . . . . . . 8
9. Method Definition . . . . . . . . . . . . . . . . . . . . . 9
10. Example Messages . . . . . . . . . . . . . . . . . . . . . . 11
11. Security Considerations . . . . . . . . . . . . . . . . . . 13
11.1 Outbound authentication . . . . . . . . . . . . . . . . . . 13
11.2 SIPS URIs . . . . . . . . . . . . . . . . . . . . . . . . . 14
11.3 End-to-End Protection . . . . . . . . . . . . . . . . . . . 14
11.4 Replay Prevention . . . . . . . . . . . . . . . . . . . . . 14
11.5 Using message/cpim bodies . . . . . . . . . . . . . . . . . 15
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . 15
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 15
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 15
15. Normative References . . . . . . . . . . . . . . . . . . . . 16
16. Informational References . . . . . . . . . . . . . . . . . . 16
17. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 17
18. Full Copyright Statement . . . . . . . . . . . . . . . . . . 18
1. Introduction
Instant Messaging (IM) is defined as the exchange of content between
a set of participants in near real time. Generally, the content is
short text messages, although that need not be the case. Generally,
the messages that are exchanged are not stored, but this also need
not be the case. IM differs from email in common usage in that
instant messages are usually grouped together into brief live
conversations, consisting of numerous small messages sent back and
forth.
Instant messaging as a service has been in existence within intranets
and IP networks for quite some time. Early implementations include
zephyr [11], the UNIX talk application, and IRC. More recently, IM
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has been used as a service coupled with presence and buddy lists;
that is, when a friend comes online, a user can be made aware of this
and have the option of sending the friend an instant message. The
protocols for accomplishing this are all proprietary, which has
seriously hampered interoperability.
The integration of instant messaging, presence, and session-oriented
communications is very powerful. The Session Initiation Protocol
(SIP) [1] provides mechanisms that are useful for presence
applications, and for session-oriented communication applications,
but not for instant messages.
This document proposes an extension method for SIP called the MESSAGE
method. MESSAGE requests normally carry the instant message content
in the request body.
RFC 2778 [3] and RFC 2779 [2] give a model and requirements for
presence and instant messaging protocols. Implementations of the
MESSAGE method SHALL support all the instant message requirements in
RFC 2779 relevant to its scope of applicability.
2. Scope of Applicability
This document describes the use of the MESSAGE method for sending
instant messages using a metaphor similar to that of a two-way pager
or SMS enabled handset. That is, there are no explicit association
between messages. Each IM stands alone--any sense of a
"conversation" only exists in the client user interface, or perhaps
in the user's own imagination. We contrast this with a "session"
model, where there is an explicit conversation with a clear beginning
and end. In the SIP environment, an IM session would be a media
session initiated with an INVITE transaction and terminated with a
BYE transaction.
There is value in each model. Most modern IM clients offer both user
experiences. The user can choose to send an IM to a contact, or he
can choose to invite one or more contacts to join a conversation.
The pager model makes sense when the user wishes to send a small
number of short IMs to a single (or small number of) recipients. The
session model makes sense for extended conversations, joining chat
groups, if there is a need to associate a conversation with some
other SIP initiated session, etc.
This document addresses the pager model only. We recognize the value
of the session model as well, but we do not define it here. Such a
solution will require additional work beyond that of this document.
The SIMPLE work group currently plans to address IM sessions in a
separate document.
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There may be a temptation to simulate a session of IMs by initiating
a dialog, then sending MESSAGE requests in the context of that
dialog. This is not an adequate solution for IM sessions, in that
this approach forces the MESSAGE requests to follow the same network
path as any other SIP requests, even though the MESSAGE requests
arguably carry media rather than signaling. IM applications are
typically high volume, and we expect the IM volume in sessions to be
even higher. This will likely cause congestion problems if sent over
a transport without congestion control, and there is no clear
mechanism in SIP to prevent some hop from forwarding a MESSAGE
request over UDP.
Additionally, MESSAGE requests sent over an existing dialog must, by
the nature of SIP, go to the same destination as any other request
sent in that dialog. This prevents any separation between the IM
endpoint and the signaling endpoint. This is not an acceptable
limitation for the session-model of instant messaging.
The authors recognize that there may be valid reasons to send MESSAGE
requests in the context of a dialog. For example, one participant in
a voice session may wish to send an IM to another participant, and
associate that IM with the session. But implementations SHOULD NOT
create dialogs for the primary purpose of associating MESSAGE
requests with one another.
Note that this statement does not prohibit using SIP to initiate a
media session made up of IMs, just like any other session. Indeed,
we expect the solution for IM sessions to use that metaphor. The
reader should avoid confusing the concepts of a SIP dialog and a
media session.
3. Overview of Operation
When one user wishes to send an instant message to another, the
sender formulates and issues a SIP request using the new MESSAGE
method defined by this document. The Request-URI of this request
will normally be the "address of record" for the recipient of the
instant message, but it may be a device address in situations where
the client has current information about the recipient's location.
For example, the client could be coupled with a presence system that
supplies an up to date device contact for a given address of record.
The body of the request will contain the message to be delivered.
This body can be of any MIME type, including message/cpim [7]. Since
the message/cpim format is expected to be supported by other instant
message protocols, endpoints using different IM protocols, but
otherwise supporting message/cpim body types, should be able to
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exchange messages without modification of the content by a gateway or
other intermediary. This helps to enable end-to-end security between
endpoints that use different IM protocols.
The request may traverse a set of SIP proxies, using a variety of
transports, before reaching its destination. The destination for
each hop is located using the address resolution rules detailed in
the Common Profile for Instant Messaging (CPIM) [8] and SIP
specifications. During traversal, each proxy may rewrite the request
URI based on available routing information.
Provisional and final responses to the request will be returned to
the sender as with any other SIP request. Normally, a 200 OK
response will be generated by the user agent of the request's final
recipient. Note that this indicates that the user agent accepted the
message, not that the user has seen it.
MESSAGE requests do not establish dialogs.
4. UAC Processing
Unless stated otherwise in this document, MESSAGE requests and
associated responses are constructed according to the rules in
section 8.1 of the SIP specification [1].
All UACs which support the MESSAGE method MUST be prepared to send
MESSAGE requests with a body of type text/plain. They may send
bodies of type message/cpim [7].
MESSAGE requests do not initiate dialogs. User Agents MUST NOT
insert Contact header fields into MESSAGE requests.
A UAC MAY associate a MESSAGE request with an existing dialog. If a
MESSAGE request is sent within a dialog, it is "associated" with any
media session or sessions associated with that dialog.
If the UAC receives a 200 OK response to a MESSAGE request, it may
assume the message has been delivered to the final destination. It
MUST NOT assume that the recipient has actually read the instant
message. If the UAC receives a 202 Accepted response, the message
has been delivered to a gateway, store and forward server, or some
other service that may eventually deliver the message. In this case,
the UAC MUST NOT assume the message has been delivered to the final
destination. If confirmation of delivery is required for a message
that has been responded to with a 202 Accepted, that confirmation
must be delivered via some other mechanism, which is beyond the scope
of this specification.
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Note that a downstream proxy could fork a MESSAGE request. If this
occurs, the forking proxy will forward one final response upstream,
even though it may receive multiple final responses. The UAC will
have no way to detect whether or not a fork occurs. Therefore the
UAC MUST NOT assume that a given final response represents the only
UAS that receives the request. For example, multiple branches of a
fork could have resulted in 2xx responses. Even though the UAC only
sees one of those responses, the request has in fact been received by
the second device as well.
The UAC MAY add an Expires header field to limit the validity of the
message content. If the UAC adds an Expires header field with a
non-zero value, it SHOULD also add a Date header field containing the
time the message is sent.
5. Use of Instant Message URIs
An instant inbox may be most generally referenced by an Instant
Message URI [8] in the form of "im:user@domain". IM URIs are
abstract, and will eventually be translated to concrete, protocol-
dependent URI.
If a UA is presented with an IM URI as the address for an instant
message, it SHOULD resolve it to a SIP URI, and place the resulting
URI in the Request-URI of the MESSAGE request before sending. If the
UA is unable to resolve the IM URI, it MAY place the IM URI in the
Request-URI, thus delegating the resolution to a downstream device
such as proxy or gateway. Performing this translation as early as
possible allows SIP proxies, which may be unaware of the im:
namespace, to route the requests normally.
MESSAGE requests also contain logical identifiers of the sender and
intended recipient, in the form of the From and To header fields.
These identifiers SHOULD contain SIP (or SIPS) URIs, but MAY include
IM URIs if the SIP URIs are not known at the time of request
construction.
Record-Route and Route header fields MUST NOT contain IM URIs. These
header fields contain concrete SIP or SIPS URIs according to the
rules of SIP [1].
6. Proxy Processing
Proxies route MESSAGE requests according to the rules of SIP [1].
Note that the MESSAGE request MAY fork; this allows for delivery of
the message to several possible terminals where the user might be. A
proxy forking a MESSAGE request follows the normal SIP rules for
forking a non-INVITE request. In particular, even if the fork
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results in multiple successful deliveries, the forking proxy will
only forward one final response upstream.
7. UAS Processing
A UAS that receives a MESSAGE request processes it following the
rules of SIP [1].
A UAS receiving a MESSAGE request SHOULD respond with a final
response immediately. Note, however, that the UAS is not obliged to
display the message to the user either before or after responding
with a 200 OK. That is, a 200 OK response does not necessarily mean
the user has read the message.
A 2xx response to a MESSAGE request MUST NOT contain a body. A UAS
MUST NOT insert a Contact header field into a 2xx response.
A UAS which is, in fact, a message relay, storing the message and
forwarding it later on, or forwarding it into a non-SIP domain,
SHOULD return a 202 (Accepted) [5] response indicating that the
message was accepted, but end to end delivery has not been
guaranteed.
A 4xx or 5xx response indicates that the message was not delivered
successfully. A 6xx response means it was delivered successfully,
but refused.
A UAS that supports the MESSAGE method MUST be prepared to receive
and render bodies of type "text/plain", and may support reception and
rendering of bodies of type "message/cpim" [7].
A MESSAGE request is said to be expired if its expiration time has
passed. The expiration time is determined by examining the Expires
header field, if present. MESSAGE requests without an Expires header
field do not expire. If the MESSAGE request containing an Expires
header field also contains a Date header field, the UAS SHOULD
interpret the Expires header field value as delta time from the Date
header field value. If the request does not contain a Date header
field, the UAS SHOULD interpret the Expires header value as delta
time from the time the UAS received the request.
If the MESSAGE expires before the UAS is able to present the message
to the user, the UAS SHOULD handle the message based on local policy.
This policy could mean: the message is deleted undisplayed,
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the message is still displayed to the user, or some other policy may
be invoked. If the message is displayed, the UAS SHOULD clearly
indicate to the user that the message has expired.
If the UAS is acting as a message relay, and is unable to deliver the
message before expiration, it chooses an action based on local
policy. This action could involve deleting the message undelivered,
delivering it as is, logging the expired message, or any other local
policy.
8. Congestion Control
Existing IM services have a history of very high volume usage.
Additionally, MESSAGE requests differ from other sorts of SIP
requests in that they carry media, in the form of IMs, as payload.
Conventional SIP payloads carry signaling information about media,
but not media itself. There is potential that when a SIP
infrastructure is shared between call signaling and instant
messaging, the IM traffic will interfere with call signaling traffic.
Congestion control in general is an issue that should be addressed at
the SIP specification level rather than for an individual method.
But since the traffic patterns are likely to be different for MESSAGE
than for most other methods, it makes sense to give MESSAGE special
consideration.
Whenever possible, MESSAGE requests SHOULD be sent over transports
that implement end-to-end congestion control, such as TCP or SCTP.
However, SIP does not provide a mechanism to prevent a downstream hop
from sending a request over UDP. Even the requirement to use TCP for
requests over a certain size can be overridden by the receiver.
Therefore use of a congestion-controlled transport by the UAC is not
sufficient.
The size of MESSAGE requests outside of a media session MUST NOT
exceed 1300 bytes, unless the UAC has positive knowledge that the
message will not traverse a congestion-unsafe link at any hop, or
that the message size is at least 200 bytes less than the lowest MTU
value found en route to the UAS. Larger payloads may be sent as part
of a media session, or using some type of content-indirection.
At the time of this writing, there is no mechanism for a UAC to
gain such knowledge outside of trivial network architectures, or
networks that are wholly controlled by a single administrative
domain. But if a mechanism for ensuring congestion-control at
each hop is created in the future, MESSAGE clients can relax the
size limit without requiring a change to this specification. The
authors expect that such a mechanism or mechanism will be created
in the near future.
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There have been discussions on making the 1300 byte limit based on
the path MTU to the next hop SIP device. The SIP specification
does exactly that, choosing the limit 200 bytes less than the path
MTU, or 1300 bytes if the device does not know the path MTU.
Transport decisions are made on a per-hop basis. However, the
point of this limit is to make sure that no upstream proxy chooses
to send a MESSAGE request with large content over UDP. Since,
except in trivial circumstances, a MESSAGE client is very unlikely
to know the MTU for upstream devices beyond the next hop, an MTU
based limit is not very useful.
A UAC MUST NOT initiate a new out-of-dialog MESSAGE transaction to a
given URI if there is a previous out-of-dialog transaction pending
for the same URI. Similarly, A UAC SHOULD NOT initiate overlapping
MESSAGE transactions inside a dialog, and MUST NOT do so unless the
route set for that dialog uses a congestion-controlled transport at
every hop.
The prohibition against overlapping MESSAGE request provides some
degree of congestion-safe behavior. A request and its associated
response must each cross the full path between the UAC and the
UAS. The time required for this will increase as networks become
congested. This provides an adaptive mechanism to slow the
introduction of new MESSAGE requests to the same destination.
It has been suggested that provisional responses should not be
allowed for pager-model MESSAGE requests. However, it is not
possible to require special treatment for MESSAGE, since many proxy
servers will not be aware of the MESSAGE method. Therefore MESSAGE
requests will receive the same provisional response treatment as any
other non-INVITE method, as described in the SIP specification.
9. Method Definition
This specification defines a new SIP method, MESSAGE. The BNF for
this method is:
MESSAGEm = %x4D.45.53.53.41.47.45 ;MESSAGE in caps
As with all other methods, the MESSAGE method name is case sensitive.
Tables 1 and 2 extend Tables 2 and 3 of SIP [1] by adding an
additional column, defining the header fields that can be used in
MESSAGE requests and responses.
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Header Field where proxy MESSAGE
__________________________________________
Accept R -
Accept 2xx -
Accept 415 m*
Accept-Encoding R -
Accept-Encoding 2xx -
Accept-Encoding 415 m*
Accept-Language R -
Accept-Language 2xx -
Accept-Language 415 m*
Alert-Info R -
Alert-Info 180 -
Allow R o
Allow 2xx o
Allow r o
Allow 405 m
Authentication-Info 2xx o
Authorization R o
Call-ID c r m
Call-Info ar o
Contact R -
Contact 1xx -
Contact 2xx -
Contact 3xx o
Contact 485 o
Content-Disposition o
Content-Encoding o
Content-Language o
Content-Length ar t
Content-Type *
CSeq c r m
Date a o
Error-Info 300-699 a o
Expires o
From c r m
In-Reply-To R o
Max-Forwards R amr m
Organization ar o
Table 1: Summary of header fields, A--O
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Header Field where proxy MESSAGE
__________________________________________
Priority R ar o
Proxy-Authenticate 407 ar m
Proxy-Authenticate 401 ar o
Proxy-Authorization R dr o
Proxy-Require R ar o
Record-Route ar -
Reply-To o
Require ar c
Retry-After 404,413,480,486 o
500,503 o
600,603 o
Route R adr o
Server r o
Subject R o
Timestamp o
To c(1) r m
Unsupported 420 o
User-Agent o
Via R amr m
Via rc dr m
Warning r o
WWW-Authenticate 401 ar m
WWW-Authenticate 407 ar o
(1): copied with possible addition of tag
Table 2: Summary of header fields, P--Z
A MESSAGE request MAY contain a body, using the standard MIME header
fields to identify the content.
10. Example Messages
An example message flow is shown in Figure 1. The message flow shows
an initial IM sent from User 1 to User 2, both users in the same
domain, "domain", through a single proxy.
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| F1 MESSAGE | |
|--------------------> | F2 MESSAGE |
| | ----------------------->|
| | |
| | F3 200 OK |
| |
RFC 3428 SIP Message Extension December 2002
Watson, come here.
The message is received by user2, displayed, and a response is
generated, message F3, and sent to the proxy:
SIP/2.0 200 OK
Via: SIP/2.0/TCP proxy.domain.com;branch=z9hG4bK123dsghds;
received=192.0.2.1
Via: SIP/2.0/TCP user1pc.domain.com;;branch=z9hG4bK776sgdkse;
received=1.2.3.4
From: sip:user1@domain.com;tag=49394
To: sip:user2@domain.com;tag=ab8asdasd9
Call-ID: asd88asd77a@1.2.3.4
CSeq: 1 MESSAGE
Content-Length: 0
Note that most of the header fields are simply reflected in the
response. The proxy receives this response, strips off the top Via,
and forwards to the address in the next Via, user1pc.domain.com, the
result being message F4:
SIP/2.0 200 OK
Via: SIP/2.0/TCP user1pc.domain.com;branch=z9hG4bK776sgdkse;
received=1.2.3.4
From: sip:user1@domain.com;;tag=49394
To: sip:user2@domain.com;tag=ab8asdasd9
Call-ID: asd88asd77a@1.2.3.4
CSeq: 1 MESSAGE
Content-Length: 0
11. Security Considerations
In normal usage, most SIP requests are used to setup and modify
communication sessions. The actual communication between
participants happens in the media sessions, not in the SIP requests
themselves. The MESSAGE method changes this assumption; MESSAGE
requests normally carry the actual communication between participants
as payload. This implies that MESSAGE requests have a greater need
for security than most other SIP requests. In particular, UAs that
support the MESSAGE request MUST implement end-to-end authentication,
body integrity, and body confidentiality mechanisms.
11.1 Outbound Authentication
When local proxies are used for transmission of outbound messages,
proxy authentication, as specified in RFC 3261, is RECOMMENDED. This
is useful to verify the identity of the originator, and prevent
spoofing and spamming at the originating network.
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11.2 SIPS URIs
The SIPS URI mechanism [1] allows a UA to assert that every hop must
occur over a secure connection. This provides some level of
integrity and privacy protection. However, this requires the users
to trust that each proxy in the request path is well-behaved, that
is, they do not violate the rules for routing SIPS URIs. Also, any
unencrypted bodies are fully exposed to the proxies.
Additionally, the possibility of a forking proxy allows a MESSAGE
request to be delivered to additional endpoints without the knowledge
of the UAC. If only hop-by-hop protection is used, the users must
trust all proxies in the chain to not fork requests to unauthorized
destinations.
11.3 End-to-End Protection
When the goal is to remedy the concerns stated above, the MESSAGE
bodies MUST be secured with S/MIME. If bodies specified in future to
be carried by the MESSAGE method have different means of providing
end-to-end security, their specification MUST describe the usage.
SIP MESSAGE endpoints MUST support encryption (CMS EnvelopeData) and
S/MIME signatures (CMS SignedData).
The S/MIME algorithms are set by RFC 3369 [4]. The AES [10]
algorithm should be preferred, as it is expected that AES best suits
the capabilities of many platforms. However, an IETF specification
for this is still incomplete as of the time of this writing.
11.4 Replay Prevention
To prevent the replay of old SIP requests, all signed MESSAGE
requests and responses MUST contain a Date header field covered by
the message signature. Any message with a date older than several
minutes in the past, or which is more than several minutes in the
future, SHOULD be answered with a 400 (Incorrect Date or Time)
message, unless such messages arrive repeatedly from the same source,
in which case they MAY be discarded without sending a response.
Obviously, this replay attack prevention mechanism does not work for
devices without clocks.
Note that there are situations where an stale Date header field is
normal. For example, the MESSAGE request may have been stored in a
store and forward server while the recipient was offline. When the
recipient returns, that server might then forward the message. Final
receipt of the message would then occur some time after it was
originally sent.
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If a UAS receives a stale message that can be confirmed to have come
from a known store and forward server (perhaps over a TLS
connection), it makes sense for it to accept the message normally.
Also, if one or more stale messages arrive shortly after an offline
period, the UAS MAY accept the message, but SHOULD warn the user that
there is a risk the message has been replayed.
11.5 Using message/cpim Bodies
The message/cpim format [7] allows for the S/MIME protection of
metadata in addition to the message payload itself. In many cases,
this metadata is redundant with SIP header fields. Still,
message/cpim adds value in that the protection of metadata can extend
across protocol boundaries. For example, a signed message/cpim body
can provide sender authentication using the message/cpim From header
field, even if the message crosses a gateway to another CPIM
compliant instant message service that does not understand SIP header
fields.
12. IANA Considerations
This specification registers the MESSAGE method in the
http://www.iana.org/assignments/sip-parameters/Method registry,
according to the following information:
MESSAGE [RFC3428]
13. Contributors
The following people made substantial contributions to this work:
Bernard Aboba Microsoft
Steve Donovan dynamicsoft
Jonathan Lennox Columbia University
Dave Oran Cisco
Robert Sparks dynamicsoft
Dean Willis dynamicsoft
14. Acknowledgments
The authors would like to thank the following people for their
support of the concept of SIP for IM, support for this work, and for
their useful comments and insights:
Jon Peterson NeuStar
Sean Olson Microsoft
Adam Roach dynamicsoft
Billy Biggs University of Waterloo
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Stuart Barkley UUNet
Mauricio Arango SUN
Richard Shockey NeuStar
Jorgen Bjorker Hotsip
Henry Sinnreich MCI Worldcom
Ronald Akers Motorola
Torrey Searle Indigo Software
Rohan Mahy Cisco
Christian Groves Ericsson
Allison Mankin ISI
Tan Ya-Ching Siemens
15.Normative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[2] Day, M., Aggarwal, S. and J. Vincent, "Instant Messaging /
Presence Protocol Requirements", RFC 2779, February 2000.
[3] Day, M., Rosenberg, J. and H. Sugano, "A Model for Presence and
Instant Messaging", RFC 2778, February 2000.
[4] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369,
August 2002.
[5] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[6] Bradner, S., "Keywords for Use in RFC's to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
16. Informational References
[7] Atkins, D. and G. Klyne, "Common Presence and Instant Messaging
Message Format", Work in Progress.
[8] Crocker, D., Diacakis, A., Mazzoldi, F., Huitema, C., Klyne, G.,
Rose, M., Rosenberg, J., Sparks, R., Sugano, H. and J. Peterson,
"Address Resolution for Instant Messaging and Presence", Work in
Progress.
[9] Rosenberg, J. and H. Schulzrinne, "SIP Caller Preferences and
Callee Capabilities", Work in Progress.
[10] Schaad, J. and R. Housley, "Use of the AES Encryption Algorithm
and RSA-OAEP Key Transport in CMS", Work in Progress.
Campbell, et. al. Standards Track [Page 16]
RFC 3428 SIP Message Extension December 2002
[11] DellaFera, C., Eichin, M., French, R., Jedlinski, D., Kohl, J.
and W. Sommerfeld, "The Zephyr notification service", in USENIX
Winter Conference (Dallas, Texas), Feb. 1988.
17. Authors' Addresses
Ben Campbell
dynamicsoft
5100 Tennyson Parkway
Suite 1200
Plano, TX 75024
EMail: bcampbell@dynamicsoft.com
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
EMail: jdrosen@dynamicsoft.com
Henning Schulzrinne
Columbia University
M/S 0401
1214 Amsterdam Ave.
New York, NY 10027-7003
EMail: schulzrinne@cs.columbia.edu
Christian Huitema
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
EMail: huitema@microsoft.com
David Gurle
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
EMail: dgurle@microsoft.com
Campbell, et. al. Standards Track [Page 17]
RFC 3428 SIP Message Extension December 2002
18. Full Copyright Statement
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Acknowledgement
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Campbell, et. al. Standards Track [Page 18]